This invention relates generally to capacitance measuring devices and, more specifically to a three-terminal capacitance measuring system employing multiple and separately selectable capacitance probe segments for monitoring the level/density of a fluidized-bed.
In a fluidized-bed reactor or combustor, the bed is fluidized by means of passing gas (air) up through the bed at a high enough velocity that the bed is agitated and partially lifted or suspended, usually in a continual but intermittent manner, such that the dynamic motion of the bed of solid particles behaves somewhat like a liquid fluid. This effect provides good particle blending and velocity that enhances heat transfer to boiler tubes for power plant use, for example. The mixing action also improves the spatial uniformity of the reactor which involves usually not only the combustion of a hydrocarbon fuel (such as coal, oil shale, etc.), but also a reaction with limestone which is also fed into the bed to remove much of the sulfur released from the fuel.
Normally, fluidizing air as it moves up through the bed forms bubbles or voids that propagate up through the bed. The rate of formation and size of the bubbles can be controlled to some degree by the velocity of the gas in the bed and the hole pattern of the air distribution plate. If the velocity is too high, then an excessive amount of solids is carried out of the reactor; if too low, inadequate fluidization is provided.
Factors affecting optimum velocity are the bed level and density. A measure of these two variables is extremely important and maintain the bed holdup (mass) and fluidization quality.
The density of the upper bed boundary varies with time (during fluidization as a result of bubble action) and with bed height. The transition from high average density to low average density at that upper boundary can range up to several inches (typically 4-6 inches), depending upon many factors.
It has been the practice to determine the bed level by means of an overflow tube, by vertical pressure differential measurements within the bed or by a water-cooled temperature probe. The use of the overflow tube results in the loss of bed material and is not continuous or reliable. A false reading can be caused by solids spray into the tube from a bed level well below the overflow entrance as a result of high bed turbulance. It also requires manual operation.
Pressure differential measurements are difficult to calibrate because of the uncertainty of the bed density, which greatly influences the pressure, and the pressure taps tend to become plugged by the bed solids.
A cooled partially immersed temperature probe with temperature sensors spaced along its vertical length is used to sense a temperature gradient change along its length corresponding to the bed level. This method suffers from lack of sensitivity and of solids layer build-up upon the surface causing uncertainty in level readings. Dynamic observations are not possible.
Thus, there is a need for a means of continuous measurement of the bed level, or mass, and density within a fluidized-bed combustor.